The present invention relates to novel qualitative and quantitative glycosidases catalytic activity assays. More particularly, the present invention relates to a method of screening for potential anti-metastatic and anti-inflammatory agents and, most particularly, to a method of screening for potential anti-metastatic and anti-inflammatory agents using mammalian heparanase, preferably purified human recombinant heparanase, as a probe.
heparan sulfate proteoglycans (HSPGs): HSPGs are ubiquitous macromolecules associated with the cell surface and extracellular matrix (ECM) of a wide range of cells of vertebrate and invertebrate tissues (1-5). The basic HSPG structure consists of a protein core to which several linear heparan sulfate chains are covalently attached. The polysaccharide chains are typically composed of repeating hexuronic and D-glucosamine disaccharide units that are substituted to a varying extent with N- and O-linked sulfate moieties and N-linked acetyl groups (1-5). Studies on the involvement of ECM molecules in cell attachment, growth and differentiation revealed a central role of HSPGs in embryonic morphogenesis, angiogenesis, metastasis, neurite outgrowth and tissue repair (1-5). The heparan sulfate (HS) chains, which are unique in their ability to bind a multitude of proteins, ensure that a wide variety of effector molecules cling to the cell surface (4-6). HSPGs are also prominent components of blood vessels (3). In large vessels they are concentrated mostly in the intima and inner media, whereas in capillaries they are found mainly in the subendothelial basement membrane where they support proliferating and migrating endothelial cells and stabilize the structure of the capillary wall. The ability of HSPGs to interact with ECM macromolecules such as collagen, laminin and fibronectin, and with different attachment sites on plasma membranes suggests a key role for this proteoglycan in the self-assembly and insolubility of ECM components, as well as in cell adhesion and locomotion. Cleavage of HS may therefore result in disassembly of the subendothelial ECM and hence may play a decisive role in extravasation of normal and malignant blood-borne cells (7-9). HS catabolism is observed in inflammation, wound repair, diabetes, and cancer metastasis, suggesting that enzymes which degrade HS play important roles in pathologic processes.
Involvement of heparanase in tumor cell invasion and metastasis: Circulating tumor cells arrested in the capillary beds of different organs must invade the endothelial cell lining and degrade its underlying basement membrane (BM) in order to escape into the extravascular tissue(s) where they establish metastasis (10). Several cellular enzymes (e.g., collagenase IV, plasminogen activator, cathepsin B, elastase) are thought to be involved in degradation of the BM (10). Among these enzymes is an endo-.beta.-D-glucuronidase (heparanase) that cleaves HS at specific intrachain sites (7, 9, 11-12). Expression of a HS degrading heparanase was found to correlate with the metastatic potential at mouse lymphoma (11), fibrosarcoma and melanoma (9) cells. The same is true for human breast, bladder and prostate carcinoma cells (see U.S. pat application Ser. No. 09/109,386, which is incorporated by reference as if fully set forth herein). Moreover, elevated levels of heparanase were detected in sera (9) and urine (U.S. patent application Ser. No. 09/109,386,) of metastatic tumor bearing animals and cancer patients and in tumor biopsies (12). Treatment of experimental animals with heparanase alternative substrates and inhibitor (e.g., non-anticoagulant species of low MW heparin, laminarin sulfate) markedly reduced (&gt;90%) the incidence of lung metastases induced by B16 melanoma, Lewis lung carcinoma and mammary adenocarcinoma cells (8, 9, 13), indicating that heparanase inhibitors may be applied to inhibit tumor cell invasion and metastasis.
Our studies on the control of tumor progression by its local environment, focus on the interaction of cells with the extracellular matrix (ECM) produced by cultured corneal and vascular endothelial cells (EC) (14, 15). This ECM closely resembles the subendothelium in vivo in its morphological appearance and molecular composition. It contains collagens (mostly type III and IV, with smaller amounts of types I and V), proteoglycans (mostly heparan sulfate- and dermatan sulfate-proteoglycans, with smaller amounts of chondroitin sulfate proteoglycans), laminin, fibronectin, entactin and elastin (13, 14). The ability of cells to degrade HS in the ECM was studied by allowing cells to interact with a metabolically sulfate labeled ECM, followed by gel filtration (Sepharose 6B) analysis of degradation products released into the culture medium (11). While intact HSPG are eluted next to the void volume of the column (Kav&lt;0.2, Mr of about 0.5.times.10.sup.6), labeled degradation fragments of HS side chains are eluted more toward the Vt of the column (0.5&lt;kav&lt;0.8, Mr of about 5-7.times.10.sup.3) (11). Compounds which efficiently inhibit the ability of heparanase to degrade the above-described naturally produced basement membrane-like substrate, were also found to inhibit experimental metastasis in mice and rats (8, 9, 13, 33). A reliable in vitro screening system for heparanase inhibiting compounds may hence be applied to identify and develop potent anti-metastatic drugs.
Possible involvement of heparanase in tumor angiogenesis: We have previously demonstrated that heparanase may not only function in cell migration and invasion, but may also elicit an indirect neovascular response (15). Our results suggest that the ECM HSPGs provide a natural storage depot for PFGF and possibly other heparin-binding growth promoting factors. Heparanase mediated release of active .beta.FGF from its storage within ECM may therefore provide a novel mechanism for induction of neovascularization in normal and pathological situations (6, 18).
Expression of heparanase by cells of the immune system: Heparanase catalytic activity correlates with the ability of activated cells of the immune system to leave the circulation and elicit both inflammatory and autoimmune responses. Interaction of platelets, granulocytes, T and B lymphocytes, macrophages and mast cells with the subendothelial ECM is associated with degradation of heparan sulfate (HS) by heparanase catalytic activity (7). The enzyme is released from intracellular compartments (e.g., lysosomes, specific granules) in response to various activation signals (e.g., thrombin, calcium ionophore, immune complexes, antigens, mitogens), suggesting its regulated involvement and presence in inflammatory sites and autoimmune lesions. Heparan sulfate degrading enzymes released by platelets and macrophages are likely to be present in atherosclerotic lesions (16). Treatment of experimental animals with heparanase alternative substrates (e.g., non-anticoagulant species of low molecular weight heparin) markedly reduced the incidence of experimental autoimmune encephalomyelitis (EAE), adjuvant arthritis and graft rejection (7, 17) in experimental animals, indicating that heparanase inhibitors may be applied to inhibit autoimmune and inflammatory diseases (7, 17). A reliable in vitro screening system for heparanase inhibiting compounds may hence be applied to identify and develop non-toxic anti-inflammatory drugs for the treatment of multiple sclerosis and other inflammatory diseases.
Cloning and expression of the heparanase gene: A purified fraction of heparanase isolated from human hepatoma cells was subjected to tryptic digestion. Peptides were separated by high pressure liquid chromatography and micro sequenced. The sequence of one of the peptides was used to screen data bases for homology to the corresponding back translated DNA sequence. This procedure led to the identification of a clone containing an insert of 1020 base pairs (bp) which included an open reading frame of 963 bp followed by 27 bp of 3' untranslated region and a poly A tail. The new gene was designated hpa. Cloning of the missing 5' end of hpa was performed by PCR amplification of DNA from placenta cDNA composite. The entire heparanase cDNA was designated phpa. The joined cDNA fragment contained an open reading frame which encodes a polypeptide of 543 amino acids with a calculated molecular weight of 61,192 daltons. Cloning an extended 5' sequence was enabled from the human SK-hep1 cell line by PCR amplification using the Marathon RACE system. The 5' extended sequence of the SK-hep1 hpa cDNA was assembled with the sequence of the hpa cDNA isolated from human placenta. The assembled sequence contained an open reading frame which encodes a polypeptide of 592 amino acids with a calculated molecular weight of 66,407 daltons. The cloning procedures are described in length in U.S. patent application Ser. No. 09/109,386.
The ability of the hpa gene product to catalyze degradation of heparan sulfate (HS) in vitro was examined by expressing the entire open reading frame of hpa in High five and Sf21 insect cells, and the mammalian human 293 embryonic kidney cell line expression systems. Extracts of infected cells were assayed for heparanase catalytic activity. For this purpose, cell lysates were incubated with sulfate labeled, ECM-derived HSPG (peak I), followed by gel filtration analysis (Sepharose 6B) of the reaction mixture. While the substrate alone consisted of high molecular weight material, incubation of the HSPG substrate with lysates of cells infected with hpa containing virus resulted in a complete conversion of the high molecular weight substrate into low molecular weight labeled heparan sulfate degradation fragments (U.S. Pat. application Ser. No. 09/109,386).
In subsequent experiments, the labeled HSPG substrate was incubated with the culture medium of infected High Five and Sf21 cells. Heparanase catalytic activity, reflected by the conversion of the high molecular weight HSPG substrate into low molecular weight HS degradation fragments, was found in the culture medium of cells infected with the pFhpa virus, but not the control pF1 virus.
Altogether, these results indicate that the heparanase enzyme is expressed in an active form by cells infected with Baculovirus or mammalian expression vectors containing the newly identified human hpa gene.
In other experiments, we have demonstrated that the heparanase enzyme expressed by cells infected with the pFhpa virus is capable of degrading HS complexed to other macromolecular constituents (e.g., fibronectin, laminin, collagen) present in a naturally produced intact ECM (09/109,386), in a manner similar to that reported for highly metastatic tumor cells or activated cells of the immune system (7, 8)
Purification of the recombinant heparanase enzyme: Sf21 insect cells were infected with pFhpa virus and the culture medium was applied onto a heparin-Sepharose column. Fractions were eluted with a salt gradient (0.35-2.0 M NaCl) and tested for heparanase catalytic activity and protein profile (SDS/PAGE followed by silver staining). Heparanase catalytic activity correlated with the appearance of a about 63 kDa protein band in fractions 19-24, consistent with the expected molecular weight of the hpa gene product. Active fractions eluted from heparin-Sepharose were pooled, concentrated and applied onto a Superdex 75 FPLC gel filtration column. Aliquots of each fraction were tested for heparanase catalytic activity and protein profile. A correlation was found between the appearance of a major protein (approximate molecular weight of 63 kDa) in fractions 4-7 and heparanase catalytic activity. This protein was not present in medium conditioned by control non-infected Sf21 cells subjected to the same purification protocol. Recently, an additional purification protocol was applied, using a single step chromatography with source-S ion exchange column. This purification resulted in a purified protein to a degree of 90%. Further details concerning theses purification procedure are disclosed in U.S. patent applicaiton Ser. Nos. 09/109,386 and 09/071,618, both are incorporated by reference as if fully set forth herein.
Recombinant heparanase for screening purposes: Research aimed at identifying and developing inhibitors of heparanase catalytic activity has been handicapped by the lack of a consistent and constant source of a purified and highly active heparanase enzyme and of a reliable screening system. Our recent cloning, expression and purification of the human heparanase-encoding gene offer, for the first time, a most appropriate and reliable source of active recombinant enzyme for screening of anti-heparanse antibodies and compounds which may inhibit the enzyme and hence be applied to identify and develop drugs that may inhibit tumor metastasis, autoimmune and inflammatory diseases.
Screening for specific inhibitors using a combinatorial library: A new approach aimed at rational drug discovery was recently developed for screening for specific biological activities. According to the new approach, a large library of chemically diversed molecules are screened for the desired biological activity. The new approach has become an effective and hence important tool for discovery of new drugs. The new approach is based on "combinatorial" synthesis of a diverse set of molecules in which several components predicted to be associated with the desired biological activity are systematically varied. The advantage of a combinatorial library over the alternative use of natural extracts for screening for desired biologically active compounds is that all the components comprising the library are known in advance (50).
In combinatorial screening, the number of hits discovered is proportional to the number of molecules tested. This is true even when knowledge concerning the target is unavailable. The large number of compounds, which may reach thousands of compounds tested per day, can only be screened, provided that a suitable assay involving a high throughput screening technique, in which laboratory automation and robotics may be applied, exists.
Prior art heparanase catalytic activity assays: Several methods for determining heparanase catalytic activity have been developed throughout the years. All of the different methods are based on radiolabeling of a substrate (either in vitro or metabolically, as described above) and analysis of its degradation products released due to heparanase catalytic activity. These prior art methods suffer several disadvantages and limitations as follows.
First, the measurement of catalytic activity is qualitative and not quantitative. This is due to the following reasons (i) the radioactive labeling is not spread evenly along the substrate chain, therefore, radioactivity may not correlate precisely with activity; (ii) since heparanase substrates are long substrate chains, a released product can be, in fact, a substrate of heparanase, however while executing any of the prior art methods, cleavage events of released products are not monitorable. Moreover, multiple cleavage events of small portions of the substrate chain are indistinguishable from fewer cleavage events, yet of longer substrate chains. Thus, not all, and in many cases, depending on the substrate chain length, not even most, of the cleavage events catalyzed by the enzyme are detectable, thereby affecting the linearity of the assay.
Second, the prior art methods are cumbersome, time-consuming and do not allow activity determination of a large number of samples simultaneously. In most cases, both preparation of the radiolabeled substrate and separation of the degradation products from the uncleaved substrate involve long and complex procedures and handling with radioactive material which calls strict safety procedures.
Third, all of the prior art methods for determining heparanase catalytic activity involve modification of the substrate by either iodination at glucosamine residues, or either O- or N-acetylation of the partially de-N-sulfated substrate. Such procedures may result in masking heparanase cleavage sites, or alternatively creating new heparanase sites.
The different prior art methods also have specific disadvantages specifically associated with each of which. Some methods involve biosynthetic radiolabeling of ECM associated HSPG and detection of HS chain degradation by gel filtration analysis of the radiolabeled material released from the labeled ECM (7, 37). In these assays, detection of the products requires a synergistic activity of proteases and heparanase. Protease is required to expose HS chains to cleavage by heparanase.
Other methods involve immobilizing chemically or biosynthetically radiolabeled heparanase substrate chains (38, 39, 40). The main disadvantage of these methods is that the immobilized substrate may be less accessible to the enzyme.
In the heparanase catalytic activity assay recently developed by Freeman and Parish (41) the products are separated from the substrate by binding to chicken histidine-rich glycoprotein (cHRG) sepharose. In this method only the lowest molecular weight products that lose the ability to bind to cHRG sepharose are detectable, while other, longer, products bind to the column with the substrate and are therefore excluded.
The mechanism by which heparanase operates on its substrate is still unknown and it is possible that some chains may first be cleaved to longer chains and then further be degraded to smaller fragments, while other chains may be directly cleaved at the end of thereof to form small fragments. The method by Freeman and Parish, therefore, fails to detect all of the cleavage products and therefore, like all of the other prior art methods for assaying heparanase catalytic activity, it is qualitative rather than quantitative.
The lack of a quantitative heparanase catalytic activity assay combined with the time and labor required to analyze a single sample using the qualitative prior art methods highlights the need for a rapid quantitative heparanase catalytic activity assay capable of assaying a large number of samples simultaneously.